1. Field of the Invention
The present invention relates to an adapter optical system for endoscope which is used by being arranged at the object side of an objective optical system for endoscope and an endoscope.
2. Description of the Related Art
So far, an endoscope has been used for observation of a part which is difficult to observe from the outside in cases such as a medical treatment and/or diagnosis of inside of body of a patient in the field of medicine, an inspection of inside of a pore formed in a product in the field of industry, etc.
Generally, an endoscope has an image transmission optical system, an objective optical system, a relay lens (in case of a rigid endoscope) an image guide fiber (in case of a flexible endoscope) etc., in the inside of an insertion part at a top part having a cylindrical shape with small diameter. And, it is constituted so that light which comes from an observation object and passes through these optical systems may be observed as an observation image through an eyepiece optical system or an image pick-up optical system. In a video endoscope, it is constituted that imaging elements, such as an objective optical system and CCD, are built in at the top part.
In General, an objective optical system for endoscope is constituted with what is called retro focus type, in order to satisfy requirements for small diameter of a path, and wide angle of an observation visual field, wherein in the order from an object side, it has a lens group with negative refracting power, a brightness aperture stop, and a lens group with positive refracting power, and an incidence pupil with a brightness aperture stop is arranged at the inside of it.
Such objective optical system for endoscope is shown in Publication of the Japanese unexamined patent application, Toku Kai. No. 2004-264835, Publication of the Japanese unexamined patent application, Toku Kai. No. 2000-10024, Japanese published unexamined patent application Toku Kai Hei 9-248276, etc., for examples.
In case that a diameter of a pipe is smaller than an outside diameter of an endoscope, for example, in case of a ureter, a bile duct and the likes which are connected with a kidney from a bladder in a living body as shown in FIG. 1, and in case that a diameter of a pore is smaller than an outside diameter of an endoscope in an industrial commodity, it is difficult to insert an endoscope having a diameter with general size into such pipe or pore. Use of an endoscope having especially a small diameter for inserting in a pore of a thin path with small diameter may be considered. However, since such endoscope with small diameter especially made requires a very high manufacturing cost, it is difficult to be used widely. Even if small-sizing or thinning of the diameter of an endoscope could be achieved, there is a certain limit. Thus, it is desired that pathological change and abnormality of a pipe and a side wall of a pore having a thin path with a diameter smaller than an outside diameter of the endoscope can be observed exactly to a certain extent.
However, in a constitution having an incidence pupil of an objective optical system for endoscope inside of an objective optical system for endoscope like the conventional endoscope, flux of light from an object enters into a top surface of the objective optical system for endoscope so that it may direct toward the central position of the incidence pupil.
In case that a pore having a thinner path than the outside diameter of an endoscope is observed, in order to observe an inside of the pore as much as possible, it is necessary to take a large angle of incidence of light which enters into the top surface of the objective optical system for endoscope from the side wall inside the pore as much as possible. For that purpose, it is necessary to bring the top surface of the objective optical system for endoscope close to an entrance of the pore as much as possible. However, even if it makes it such way, if the path of the pore is smaller than the path of the top surface of the objective optical system for endoscope, the light from the perimeter of the entrance of the pore in addition to the light from the side wall of the pore will enter into the top surface of the objective optical system for endoscope at a considerable rate. And, since incidence light from the smaller pore than the diameter of the objective lens has a smaller angle of incidence than the incidence light from the perimeter of the pore, an amount of information of the image obtained from the side wall of the pore becomes less.